Experimental study of the Talbot effect under femtosecond laser illumination

Experimental results of the Talbot effect of an amplitude grating under femtosecond laser illumination are reported. Compared with Talbot image under continuous wave (CW) illumination, Talbot images under femtosecond laser illumination are different due to the wide spectral bandwidth and the Talbot images are more distorted at longer Talbot distances. The spectrums and the pulsewidths of femtosecond laser pulses are measured with the frequency-resolved optical gating (FROG) apparatus. Experimental results are in good agreement with the theoretical analysis. (c) 2005 Elsevier B.V. All rights reserved.

Cr^4＋：YAG被动调QNd：YAG陶瓷激光器的研究

介绍了近几年迅速发展的一种新型激光介质——透明Nd：YAG多晶陶瓷的发展状况，对比分析了多晶陶瓷与单晶的光谱特性、激光特性和连续实验研究情况。并对钛宝石激光器调谐至808nm，端面抽运Nd：YAG陶瓷被动调Q全固态激光器的脉冲运转进行了较为详细的理论分析和实验研究。采用初始透射率为90％的Cr^4＋：YAG可饱和吸收晶体，被动调Q的阈值功率为119mW，当端面抽运功率为465mW时，获得波长为1064nm，脉宽为16ns，重复频率为18．18kHz，单脉冲能量为3．4μJ，平均输出功率为61mW的稳定调Q

高重复频率、窄脉宽全固态光纤放大器种子源

高重复频率、窄脉宽的全固态激光器种子源级联光纤放大器是获得高功率脉冲激光输出的有效手段.短上能态寿命的Nd∶YVO4晶体在连续抽运、高重复频率Q开关工作时容易得到接近连续性能的平均输出功率.理论分析了声光(AO)调Q器件中影响输出能量和脉宽大小的主要因素,优化配置了腔型参数.利用激光二极管(LD)光纤耦合模块端面抽运Nd∶YVO4晶体,实现了声-光调Q重复频率100 kHz以上,脉宽20 ns以下,波长1064 nm的激光输出.在抽运功率5.7 W时,得到了脉宽15.3 ns,重复频率150 kHz的种子光输出,在级联单级光纤放大器后,得到了20 W的输出.

铒镱共掺磷酸盐玻璃的激光性质

制备了镱铒共掺的磷酸盐玻璃并研究了室温下LD泵浦的连续激光输出性质。在泵浦功率为496mW时实现了最大输出功率77mW。讨论了在不同玻璃样品厚度和谐振腔长度时的斜率效率的变化以及在不同玻璃样品厚度，泵浦功率和谐振腔长度时的不同激光模式竞争的动力学行为。结果表明：激光光谱受到光学增益和光学损耗相对大小的限制。

Dynamical behavior of damped driven coupled single electron simple harmonic oscillators

Coherent coupling between a large number of qubits is the goal for scalable approaches to solid state quantum information processing. Prototype systems can be characterized by spectroscopic techniques. Here, we use pulsed-continuous wave microwave spectroscopy to study the behavior of electrons trapped at defects within the gate dielectric of a sol-gel-based high-k silicon MOSFET. Disorder leads to a wide distribution in trap properties, allowing more than 1000 traps to be individually addressed in a single transistor within the accessible frequency domain. Their dynamical behavior is explored by pulsing the microwave excitation over a range of times comparable to the phase coherence time and the lifetime of the electron in the trap. Trap occupancy is limited to a single electron, which can be manipulated by resonant microwave excitation and the resulting change in trap occupancy is detected by the change in the channel current of the transistor. The trap behavior is described by a classical damped driven simple harmonic oscillator model, with the phase coherence, lifetime and coupling strength parameters derived from a continuous wave (CW) measurement only. For pulse times shorter than the phase coherence time, the energy exchange between traps, due to the coupling, strongly modulates the observed drain current change. This effect could be exploited for 2-qubit gate operation. The very large number of resonances observed in this system would allow a complex multi-qubit quantum mechanical circuit to be realized by this mechanism using only a single transistor.

Plume attenuation under high power Nd:yttritium-aluminum-garnet laser welding

During high-power continuous wave (cw) Nd:yttritium-aluminum-garnet (YAG) laser welding a vapor plume is formed containing vaporized material ejected from the keyhole. The gas used as a plume control mechanism affects the plume shape but not its temperature, which has been found to be less than 3000 K, independent of the atmosphere and plume control gases. In this study high-power (up to 8 kW) cw Nd:YAG laser welding has been performed under He, Ar, and N2 gas atmospheres, extending the power range previously studied. The plume was found to contain very small evaporated particles of diameter less than 50 nm. Rayleigh and Mie scattering theories were used to calculate the attenuation coefficient of the incident laser power by these small particles. In addition the attenuation of a 9 W Nd:YAG probe laser beam, horizontally incident across the plume generated by the high-power Nd:YAG laser, was measured at various positions with respect to the beam-material interaction point. Up to 40% attenuation of the probe laser power was measured at positions corresponding to zones of high concentration of vapor plume, shown by high-speed video measurements. These zones interact with the high-power Nd:YAG laser beam path and, can result in significant laser power attenuation. © 2004 Laser Institute of America.

High-performance 2 mm gate width GaNHEMTs on 6H-SiC with output power of 22.4 W @ 8 GHz

Optimized AlGaN/AlN/GaN high electron mobility transistors (HEMTs) structures were grown on 2-in semi-insulating (SI) 6H-SiC substrate by metal-organic chemical vapor deposition (MOCVD). The 2-in. HEMT wafer exhibited a low average sheet resistance of 305.3 Omega/sq with a uniformity of 3.85%. The fabricated large periphery device with a dimension of 0.35 pm x 2 nun demonstrated high performance, with a maximum DC current density of 1360 mA/mm, a transconductance of 460 mS/mm, a breakdown voltage larger than 80 V, a current gain cut-off frequency of 24 GHz and a maximum oscillation frequency of 34 GHz. Under the condition of continuous-wave (CW) at 9 GHz, the device achieved 18.1 W output power with a power density of 9.05 W/mm and power-added-efficiency (PAE) of 36.4%. While the corresponding results of pulse condition at 8 GHz are 22.4 W output power with 11.2 W/mm power density and 45.3% PAE. These are the state-of-the-art power performance ever reported for this physical dimension of GaN HEMTs based on SiC substrate at 8 GHz. (c) 2008 Elsevier Ltd. All rights reserved.

An internally-matched GaN HEMTs device with 45.2 W at 8 GHz for X-band application

Optimized AlGaN/AlN/GaN high electron mobility transistor (HEMT) with high mobility GaN channel layer structures were grown on 2-in. diameter semi-insulating 6H-SiC substrates by MOCVD. The 2-in. diameter GaN HEMT wafer exhibited a low average sheet resistance of 261.9 Omega/square, with the resistance un-uniformity as low as 2.23%. Atomic force microscopy measurements revealed a smooth AlGaN surface whose root-mean-square roughness is 0.281 nm for a scan area of 5 x 5 mu m. For the single-cell HEMTs device of 2.5-mm gate width fabricated using the materials, a maximum drain current density of 1.31 A/mm, an extrinsic transconductance of 450 mS/mm, a current gain cutoff frequency of 24 GHz and a maximum frequency of oscillation 54 GHz were achieved. The four-cell internally-matched GaN HEMTs device with 10-mm total gate width demonstrated a very high output power of 45.2 W at 8 GHz under the condition of continuous-wave (CW), with a power added efficiency of 32.0% and power gain of 6.2 dB. To our best knowledge, the achieved output power of internally-matched devices are the state-of-the-art result ever reported for X-band GaN-based HEMTs. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

A novel photonic crystal vertical cavity surface emitting laser based on coherent coupling

We demonstrate a novel oxide confined GaAs-based photonic crystal vertical cavity surface emitting laser (PC-VCSEL) operating at a wavelength of 850 nm based on coherent coupling. A ring-shaped light-emitting aperture is added to the conventional PC-VCSEL, and coherent coupling is achieved between the central defect aperture and the ring-shaped light-emitting aperture. Measurements show that under the continuous-wave (CW) injected current of 20 mA, a high power of 2 mW is obtained, and the side mode suppression ratio (SMSR) is larger than 20 dB. The average divergence angle is 4.2 degrees at the current level of 20 mA. Compared with the results ever reported, the divergence angle is reduced.

5.3-W Nd:YVO_4 passively mode-locked laser by a novel semiconductor saturable absorber mirror

We report a diode end-pumped continuous wave (CW) passively mode-locked Nd:YVO4 laser with a homemade semiconductor saturable absorber mirror (SESAM). The maximum average output power is 5.3 W at the incident pump power of 17 W, which corresponds to an optical-optical conversion efficiency of 31.2% and slope efficiency of 34.7%. The corresponding optical spectrum has a 0.2-nm full width at half maximum (FWHM), and the pulse repetition rate is 83 MHz.

Retrodirective Antenna Spatial Data Protection

In this letter, we show how a 2.4-GHz retrodirective array operating in a multipath rich environment can be utilized in order to spatially encrypt digital data. For the first time, we give experimental evidence that digital data that has no mathematical encryption applied to it can be successfully recovered only when it is detected with a receiver that is polarization-matched to that of a reference continuous-wave (CW) pilot tone signal. In addition, we show that successful detection with low bit error rate (BER) will only occur within a highly constrained spatial region colocated close to the position of the CW reference signal. These effects mean that the signal cannot be intercepted and its modulated data recovered at locations other than the constrained spatial region around the position from which the retrodirective communication was initiated.

Amplitude-modulation characteristics of power-combining Class-E amplifier with finite choke

Power back-off performances of a new variant power-combining Class-E amplifier under different amplitude-modulation schemes such as continuous wave (CW), envelope elimination and restoration (EER), envelope tracking (ET) and outphasing are for the first time investigated in this study. Finite DC-feed inductances rather than massive RF chokes as used in the classic single-ended Class-E power amplifier (PA) resulted from the approximate yet effective frequency-domain circuit analysis provide the wherewithal to increase modulation bandwidth up to 80% higher than the classic single-ended Class-E PA. This increased modulation bandwidth is required for the linearity improvement in the EER/ET transmitters. The modified output load network of the power-combining Class-E amplifier adopting three-harmonic terminations technique relaxes the design specifications for the additional filtering block typically required at the output stage of the transmitter chain. Qualitative agreements between simulation and measurement results for all four schemes were achieved where the ET technique was proven superior to the other schemes. When the PA is used within the ET scheme, an increase of average drain efficiency of as high as 40% with respect to the CW excitation was obtained for a multi-carrier input signal with 12 dB peak-to-average power ratio. © 2011 The Institution of Engineering and Technology.

Pilot tone reference-less phase conjugator for phase modulated retrodirective antenna applications

This letter presents a simple tracking phased locked loop (PLL) that can be used to track phase-modulated signals and provide a phase-conjugated signal for retrodirective retransmission. The configuration allows the retrodirective antenna to directly track phase-modulated signals with no requirement for a separate continuous wave (CW) pilot tone. The ability to directly track phase-modulated signals is carried out using a 4× multiplier on the tracking PLL reference signal. Practical phase conjugation results are presented for a five-element retrodirective array simultaneously sending and receiving phase-modulated (QPSK) signals. Signals with levels as low as -122 dBm can be phase-conjugated and retransmitted with 30 dBm EIRP.

Modifications of Surface Properties of Beta Ti by Laser Gas Diffusion Nitriding

Beta-type Ti-alloy is a promising biomedical implant material as it has a low Young’s modulus and is also known to have inferior surface hardness. Various surface treatments can be applied to enhance the surface hardness. Physical vapor deposition and chemical vapor deposition are two examples of this but these techniques have limitations such as poor interfacial adhesion and high distortion. Laser surface treatment is a relatively new surface modification method to enhance the surface hardness but its application is still not accepted by the industry. The major problem of this process involves surface melting which results in higher surface roughness after the laser surface treatment. This paper will report the results achieved by a 100 W continuous wave (CW) fiber laser for laser surface treatment without the surface being melted. Laser processing parameters were carefully selected so that the surface could be treated without surface melting and thus the surface finish of the component could be maintained. The surface and microstructural characteristics of the treated samples were examined using x-ray diffractometry, optical microscopy, three-dimensional surface profile and contact angle measurements, and nanoindentation test.

Magnetic resonance and conductivity study of gelatin-based proton conductor polymer electrolytes

This work report results from proton nuclear magnetic resonance (NMR), continuous-wave (CW-EPR) and pulsed electron paramagnetic resonance (P-EPR) and complex impedance spectroscopy of gelatin-based polymer gel electrolytes containing acetic acid. cross-linked with formaldehyde and plasticized with glycerol. Ionic conductivity of 2 x 10(-5) S/cm was obtained at room temperature for samples prepared with 33 wt% of acetic acid. Proton ((1)H) line shapes and spin-lattice relaxation times were measured as a function of temperature. The NMR results show that the proton mobility is dependent on acetic acid content in the plasticized polymer gel electrolytes. The CW-EPR spectra, which were carried out in samples doped with copper perchlorate, indicate the presence of the paramagnetic Cu(2+) ions in axially distorted sites. The P-EPR technique, known as electron spin echo envelope modulation (ESEEM), was employed to show the involvement of both, hydrogen and nitrogen atoms, in the copper complexation of the gel electrolyte. (C) 2009 Elsevier Ltd. All rights reserved.